DISTINGUISHING EGG MASSES OF UNISEXUAL AND JEFFERSON SALAMANDERS

Unisexual Ambystoma are difficult to distinguish from the sexual salamanders with which they interact, and all of these species are of conservation concern throughout much of their range. We examined egg masses to develop an efficient, affordable method to differentiate unisexual salamanders and Jefferson Salamanders (A. jeffersonianum). We developed a technique based on photographs of egg masses from three populations held in the laboratory and tested it in the field. Using photographs, we first determined the long axis of the egg mass, and then drew a line perpendicular to the long axis and counted the maximum number of embryos or egg membranes that intersected with this line. The resulting Embryo Density Index was higher in egg masses of Jefferson Salamander (mean = 6.3 +/- 2.0 SD) than in unisexuals (mean = 3.8 +/- 1.3). Likewise, the Egg Membrane Density Index was higher in egg masses of Jefferson Salamanders (mean = 7.7 +/- 2.3) than in unisexuals (mean = 4.6 +/- 1.6). Although those results were statistically significant, there was considerable overlap in egg density index values between salamander biotypes, making the methods more appropriate for population-level analyses than for determining species identity of a given individual egg mass. Our scale-independent method of visually assessing the spacing of eggs within a mass can be used across developmental stages, and it enables rapid data collection with very little equipment or cost.